Conventional LCD backlighting assemblies use a linear cold cathode fluorescent lamp (CCFL) to inject light into the edge of a clear light guide. In order to extract light from the light guides in backlights, diffusing white spots are often printed on the bottom of the light guide. These spots reflectively diffuse light out of the light guide. The control over the angular spread of the reflected light from the dots is very poor; a significant amount of light is redirected back toward the lamp or other to areas where the light is absorbed. The poor control over light also directs light into wide angles and viewing zones where it is often undesired. The loss of light results in dimmer displays or lost electrical power. Minimizing electrical power drain and maximizing brightness are critical in portable and handheld devices such as laptop computers and mobile phones.
For extended color, longer lifetime, increased optical efficiency, and cost, LEDs are becoming utilized more in backlight assemblies instead of CCFLs. Since LEDs are closer to being a point source, LED light can be controlled more efficiently than the extended source CCFL. However, by using the same white spot diffusers noted above in light guides, the light is scattered in all directions, up to the critical angle of the light guide air interface. The refracted angular spread of light out of the light guide can reach angles approaching 90 degrees from the surface. Additional diffuser films used to reduce the visibility of the white dots spread this light further into undesired, i.e., wider, angles. Optical films such as prismatic films are then necessary, to “rein in” a portion of this light back toward 0 degrees (the direction perpendicular to the surface). Thus, between the white dots spreading light out into larger angles than needed, and then using collimating films to bring a portion of this light back toward the normal or desired viewing angles, a significant amount of light is lost and the process is an inefficient one.
Other backlight configurations have been proposed using symmetric scattering particles instead of white dots. Scattering light guides have been described as “highly scattering optical transmission” (HSOT) polymers by Okumura et al (J. Opt. A: Pure Appl. Opt. 5 (2003) S269-S275). The authors demonstrated that a backlight based upon a HSOT polymer has the potential to provide twice the brightness of a conventional backlight. However, the particles used are symmetric or spherical in shape. The Okumura teachings do not account for the asymmetric nature of the input light, or the need for more light to be diffused vertically, horizontally, or out from the main face of the light guide. Backlights that use symmetric diffusers also scatter light into undesired directions, and have poor optical control over the scattering.